The molding of plastic parts having one or more rib sections projecting orthogonally from a wall portion frequently involves problems of warping, sink marks and shrinkage because of the non-uniform cooling rate for the molded article. This is particularly true in instances where the rib portion is thicker than the wall portion, as is often necessary, in order to get sufficient strength of the rib section. Because of the thickness of this region, the rib section continues to cool and shrink after the wall has stopped shrinking, causing bowing or warping of the wall as well as sink marks. The removal of the molded part from the mold cavity without separating the rib portion from the wall portion also presents problems.
These problems are significant in the molding of plastic dishwasher tubs. Plastic dishwasher tubs are known in the art as exemplified by commonly-assigned U.S. Pat. Nos. 3,822,029 to Butsch, 3,826,553 to Cushing, 3,844,441 to Clark, and 4,295,692 to Jenkins. These dishwasher tubs include relatively large generally planar wall portions. Molding apparatus for forming plastic dishwasher tubs of the aforementioned type is disclosed in U.S. Pat. No. 3,905,740 to Walter R. Lovejoy. The Lovejoy apparatus works satisfactorily for tubs of the non-ribbed type. However, where it is desirable to enhance the structural rigidity of the plastic tub by adding structural support ribs on the tub walls, the problem of warped walls and sink marks becomes significant. In addition to the unsightly appearance of a dishwasher tub having warped walls and sink marks, warped walls could also cause distortion of the access opening to the tub, making a tight sealing fit of the door difficult to maintain. A method of molding such plastic articles which minimizes wall warppage and unsightly sink marks is clearly desirable.
It is therefore an object of the present invention to provide a method of manufacturing molded plastic articles such as dishwasher tubs having generally planar wall portions with extending ribs which reduces wall warppage and sink marks caused by uneven cooling of the wall and rib portions.
It is a further object of the present invention to provide improved molding apparatus for molding plastic articles such as dishwasher tubs of the aforementioned type.